Drying oils are among the oldest binders used in paints and are still used today as raw materials in alkyd resins, epoxy esters, and uralkyds. Naturally occurring drying oils are triglycerides consisting of glycerol esters of a mixture of saturated fatty acids and unsaturated fatty acids such as oleic, linoleic, and linolenic fatty acids. These triglycerides are known to react with molecular oxygen via an auto-oxidative crosslinking reaction to form a coating. Metal catalyst known as driers can be added to accelerate the drying process. The metal catalysts can promote the formation of peroxide radicals which initiate the auto-oxidative process.
Drying oils crosslink through unsaturated fatty acid residues via an auto-oxidative process. Typically, the auto-oxidative process is separated into three steps; initiation, propagation, and termination. In the initiation step, naturally present hydroperoxides decompose forming free radicals. These free radicals have high reactivity toward antioxidants which form peroxy free radicals. As the concentration of antioxidants decrease, propagation proceeds by abstraction of hydrogen atoms on methylene groups between double bonds forming free radical. These free radicals can react with oxygen forming a conjugated peroxy free radical. Regeneration of free radicals can proceed by abstracting additional hydrogen atoms from other doubly allylic methylene groups. Crosslinking can then occur by radical-radical combination forming carbon-carbon, ether and peroxide bonds.
Many attempts have been made to develop novel inorganic/organic materials. One successful method developed in the past decade involves in situ polycondensation of metal alkoxides in organic polymer matrices via a sol-gel process. The sol-gel process of metal alkoxides permits low temperature synthesis while yielding high purity homogenous ceramic-type materials. Initial hydrolysis of the metal alkoxides typically occurs when an inorganic molecule reacts with water to form a partially hydrolyzed complex. The curing process continues as molecules with various degrees of hydrolysis react via polycondensation. Ultimately, the polycondensation results in a three-dimensional mixed metal oxide/hydroxide/alkoxide clusters.
The characteristics of ceramic materials utilizing the sol-gel process have been investigated as corrosion-protective coatings on metal substrates. Numerous papers have indicated their unique ability to protect metal substrates by acting as a barrier coating. In addition, ceramic coatings have been indicated to protect metal substrates by covalent interaction with the oxide layer present at the metal substrate surface. However, it should be emphasized that until the recent development of the sol-gel process, substrates such as aluminum with a low melting point could not be coated with ceramics due to the high temperatures required by conventional methods of ceramic film formation. The sol-gel process allows one to obtain ceramic films at lower temperatures.
More recently inorganic/organic hybrid coatings have been reported utilizing drying oils and sol-gel precursors. The seed oil was mixed with a metal alkoxide and the resulting inorganic/organic hybrid coatings cured via concomitant reactions in the inorganic and organic phases. Investigation of the inorganic/organic hybrid coatings as a function of sol-gel precursor, sol-gel concentration, and type of drying oil found that hardness, and adhesion significantly increased with increasing sol-gel precursor content. Overall, the inclusion of sol-gel precursors such as Ti(O-i-Pr).sub.4 in drying oil based coatings, however, decreased impact resistance and flexibility in comparison to the corresponding linseed oil and sunflower oil coatings.